Below please find the link to the Powerpoint presentation for lesson #4 for #TUBiol3373. The lesson first competes the discussion on G Protein Coupled Receptors, including how cells terminate cell signals. Included are mechanisms of receptor desensitization. Please NOTE that desensitization mechanisms like B arrestin decoupling of G proteins and receptor endocytosis occur after REPEATED and HIGH exposures to agonist. Hydrolysis of GTP of the alpha subunit of G proteins, removal of agonist, and the action of phosphodiesterase on the second messenger (cAMP or cGMP) is what results in the downslope of the effect curve, the termination of the signal after agonist-receptor interaction.